The present invention relates to the molding of tires and, in particular, the molding of the tread of tires.
In order to assure a molding of excellent quality, numerous conditions must be satisfied. One of these conditions specifically concerns the operation of the removal from the mold of the tread which must also impose as few constraints as possible on the tire, subject otherwise to causing disturbances, in particular in the reinforcement architecture of the tire. This leads to a preference for the so-called "sector molds", illustrated for instance in U.S. Pat. No. 3,779,677, rather than the so-called "2-part" molds, such as that described for example in U.S. Pat. No. 2,874,405.
Furthermore, it is desirable that, particularly at the level of the tread, there be no burr due to rubber which has flowed between two molding elements, for instance between two sectors. For this reason, in general, the mold is closed (all sectors adjoining each other) before imposing additional shaping on the raw tire. This additional shaping is obtained by increase in the pressure prevailing in the shaping bladder, which causes the rubber of the tread to penetrate into the elements molding the tread pattern.
Now, in certain tire architectures comprising zero degree reinforcements in the crown, it is difficult or even impossible to contemplate additional shaping in a vulcanization press. The raw tire is then assembled to a size closer to or substantially equal to the size of the vulcanized molded tire. The penetration of the molding elements into the rubber can then take place before the closed position. This results in a flow of rubber in peripheral direction, which may result in substantial burrs at the planes of joints between sectors.
In order to remedy this problem, the design of the tread pattern has up to now been varied; namely the placing of wear indicators to the interfaces between sectors or a decrease in the extent of cutting of the tread patterns upon approaching the edges of sectors. This makes it possible to increase the volume of rubber of the tread pattern at these places so as to absorb small peripheral flows of rubber. These are only palliators that do not contribute to increasing the quality of the tires or treads produced.
These molding constraints result from the fact that, in a sector mold, since each sector covers a relatively large angle, the molding is purely radial only in the median zone of each sector. Upon observing the movement of advance of the sectors, it is noted that the molding is radial only at the level of the median plane of each sector. The molding takes place in a direction which forms a given angle with respect to the radial direction, which angle is larger the further one moves away, in circumferential direction from this median position, in order to reach the edges of the sectors. The maximum value of this angle corresponds in general to half the angular development of the sector.
Developing this finding for molds having numerous successive pattern units along the perimeter, as is generally true of passenger car tires, it is noted that, in so-called "sector" molds, the number of sectors remains considerably smaller than the number of successive pattern units, which leads to a relatively large circumferential size of each sector (typically one eighth of the perimeter for a passenger-car tire).
Even though this type of mold constitutes a considerable advance over the two-part molds, which are without any radial movement upon opening or closing, it cannot truly be considered that sector molds effect a radial molding of the tread. Considering one sector of the mold and assuming that it is displaced radially with respect to the axis of the mold, this means that the actual displacement noted is parallel to a radius at any point on the molding surface of the sector in question. A purely radial movement at the level of the molding surface is obtained only for a single circumferential position. The actual movement differs from the theoretical radial direction for every other point of the molding surface.
Stated differently, only the points of said molding surface which intersect the reference radius of the radial displacement in question describe a pure radial movement. All the other points describe a movement parallel to said radius, and more or less different from it.
This movement breaks down therefore into a radial component and a circumferential component, the latter being undesirable and may be considered an "induced" component. By observing on one sector, during a radial stroke corresponding substantially to the depth of the tread pattern, the size of this induced displacement which follows a circumferential component instead of a pure radial direction, it is noted that said circumferential component reaches a value on the order of 0.2% of the molded perimeter in the case of a conventional mold having eight sectors.
This induced circumferential movement causes, upon the molding, flows of rubber directed towards the circumferential edges of the sectors, that is to say, towards the interface between the adjacent sectors. This parasitic movement contributes to the appearance of molding burrs which appear on the tread of a vulcanized tire. One object of the invention is to correct these molding defects which the burrs constitute.
U.S. Pat. No. 4,895,692 describes a completely rigid mold having a peripheral ring of sectors for molding the tread, two lateral shells for molding the side walls (outer surfaces of the tire), and a rigid core for molding the inner surface of the tire. Since there is no additional shaping, the use of this mold frees the tire designer from a constraint specific to that phase of the molding. The completely rigid design of this mold results in numerous advantages as to the quality of the molded tire, since the geometrical shapes obtained are of high quality (excellent circularity, in every transverse position). However, the imposed volume molding makes it necessary to maintain very narrow tolerances with respect to the volume of the raw blanks of the tire.
Another object of the invention is to be able to retain the advantage of the rigid mold from the standpoint of respect for and perfect control of the geometrical quality of the tires manufactured with this type of mold while making the molding operation less sensitive to differences in volume between the raw blanks of the tires to be molded and vulcanized successively in the same mold.
The mold of the present invention not only makes it possible to achieve these objectives but it also contributes a general improvement to the molding and removal from the mold of the tires, whether or not use is made of a rigid core in order to mold the inner cavity of the tire.